Helical Formation for a Conduit

ABSTRACT

A helical formation within a conduit and a method of determining the helix angle of the helical formation are disclosed. The method includes specifying the internal dimensions of the conduit and an intended fluid mass flow through the conduit. The helix angle is determined from the pressure drop and the turbulent kinetic energy for a conduit having the specified internal dimensions and intended fluid mass flow.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. patent application Ser. No.10/516,875, with a U.S. filing date of Dec. 3, 2004. U.S. applicationSer. No. 10/516,875 is a 371 filing of PCT/GB02/05646, filed on Dec. 13,2002 which in turn claims priority of PCT/GB02/02580, filed on Jun. 5,2002.

Furthermore, the underlying concepts, but not necessarily the language,of U.S. patent application Ser. No. 10/516,875 are incorporated hereinby reference.

-   If there are any contradictions or inconsistencies in language    between this application and the case that have been incorporated by    reference that might affect the interpretation of the claims in this    case, the claims in this case should be interpreted to be consistent    with the language in this case.

FIELD OF INVENTION

The invention relates to a method of determining the helix angle of ahelical formation for a conduit, and in particular, but not solely, forblood flow tubing.

BACKGROUND OF THE INVENTION

A number of documents have proposed using helical formations in conduitsto encourage a desired flow pattern of a fluid within the conduit. Suchhelical formations have been proposed for a wide variety ofapplications, including pipelines and blood flow tubing. The purpose ofthe helical formations is generally to generate spiral flow of the fluidwithin the conduit to reduce turbulence and dead spots within theconduit.

Although the use of helical formations has been proposed as beneficialto fluid flow in conduits by helping to generate spiral fluid flowpatterns, there is little or no information on the physicalcharacteristics or design of the helical formation that is required tocreate a suitable spiral flow pattern. Clearly, some designs of helicalformations will be ineffective at creating spiral flow and others willnot create a beneficial spiral flow. For example, helical formationshaving a high helix angle may tend to create turbulence rather thanspiral flow due.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided a helical formation for a conduit, the helical formationdefining at least a portion of a helix, the angle of the helix definedby the helical formation being determined from the internal dimensionsof the conduit, the fluid mass flow of the conduit, the pressure dropalong the conduit and the turbulent kinetic energy within the conduit.

In accordance with a second aspect of the present invention, there isprovided a method of determining the helix angle of a helical formationfor a conduit, the method comprising specifying the internal dimensionsof the conduit and an intended fluid mass flow through the conduit, anddetermining the helix angle from the pressure drop and the turbulentkinetic energy for a conduit having the specified internal dimensionsand intended fluid mass flow.

The terms “helical”, “helix” and “spiral” as used herein cover themathematical definition of helical and any combination of themathematical definitions of helical and spiral.

Typically, the pressure drop and the turbulent kinetic energy arenon-dimensionalised before the helix angle is determined.

Preferably, the helix angle is determined as the helix angle at whichthe non-dimensionalised pressure drop and the non-dimensionalisedturbulent kinetic energy are substantially equal. However, the helixangle could be determined as a helix angle at which thenon-dimensionalised pressure drop and the non-dimensionalised turbulentkinetic energy are not equal, depending on the type of conduit, thefluid and/or the application.

The helical formation may have a helix angle of between 5° and 50°. Forexample, the helical formation may have a helix angle of about 8°,particularly but not exclusively in relation to arterial flow in legarterial grafts.

Typically, the fluid to be carried by the conduit comprises a liquid.The fluid may be solely a liquid, a liquid mixed with a particulatesolid, or a liquified solid. For example, where the conduit is a bloodvessel, the liquid is blood.

Typically, the helical formation may effect a rotational flow of fluidwithin the conduit, in use. The rotational flow may comprise a helicaland/or spiral flow component.

Preferably, the helical formation may comprise an elongate member.Typically, the elongate member comprises an inwardly extending portion.

In one example of the invention, the helical formation may be in theform of an insert adapted to be mounted permanently or temporarilywithin the conduit.

In another example of the invention, the helical formation may be anintegral part of the conduit and may be formed, for example, by adeformation of a side wall of the conduit.

The helical formation may effect helical and/or spiral flow in such afashion as to eliminate or reduce turbulence and/or eliminate or reducedead flow regions in the conduit. The helix angle to achieve such flowwill depend on such factors as diameter of the conduit, longitudinal androtational velocity of the fluid, and the viscosity and othercharacteristics of the fluid.

The conduit may comprise tubing. For example, the conduit may compriseartificial or natural blood flow tubing, such as a vascular graft or ablood vessel, respectively. The tubing may be used in blood treatment ordelivery equipment, for example a heart-lung machine, dialysis equipmentor a giving set. The tubing may also be used in industrial equipment,for example hoses, pipes or fire hoses.

Alternatively, the conduit may comprise a stent. Stents, for examplemade of mesh, expanded sheet or tube or wire spring type, are insertedinto blood vessels to provide mechanical support and prevent collapse ofthe blood vessel. A structure according to the present invention couldbe placed inside or outside the blood vessel to impose, maintain and/orreinforce a flow guiding formation through the blood vessel.

The invention may also be utilised for stent grafts. That is, acombination of stent and graft.

Flow configuration through a conduit may, in general, be measured usingsuch techniques as magnetic resonance imaging (MRI) and/or Dopplerultrasound, and the flow guiding formation may be modified accordinglyuntil a desired flow configuration is achieved. Initial design of flowconfiguration may be by mathematical modelling or by trial and error,with modification as described above.

The conduit may be a flexible conduit, such as a tube or hose, or asubstantially rigid conduit, such as a metal pipe or a pipeline.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of a method of determining the helix angle of a helicalformation will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a perspective view of an arterial graft having a helicalformation; and

FIG. 2 is a graph of helix angle versus pressure drop and helix angleversus turbulent kinetic energy for the arterial graft.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of an arterial graft 1 for implantation inthe human or animal body. The graft 1 is fabricated from a knitted orwoven polyester material. However, any suitable flexible material couldbe used, such as a spun polyurethane multi-monofilament or a PTFEextrusion.

The graft 1 is in the form of a tube 2 that has a deformation 3 in theside wall of the tube 2 so that the deformation 3 extends inwardlygenerally towards the longitudinal axis of the tube 2 to form a helicalformation 4 on the internal surface of the tube 2. The tube is alsocrimped to form circumferential ridges 5 along the length of the tube 2.The circumferential ridges help to provide radial strength to the tube 2to minimise the risk of the graft collapsing during implantation andsubsequently during use.

The helical formation 4 is intended to promote a rotational flow patternto blood passing through the graft 1, in use. It is believed thatrotational flow has beneficial effects in reducing the effect of andhelping to prevent arterial diseases, by reducing turbulent flow andreducing dead spots within the flow.

The inventors have found that the choice of the helix angle of thehelical formation 4 is important in minimising turbulent flow and deadspots within the flow. The inventors have also found that for a conduithaving given internal dimensions and a particular helical flow formationthat is intended to carry a given mass flow, the optimum helix angle canbe determined from the pressure drop along the conduit and the turbulentkinetic energy in the conduit.

In addition, the inventors have found that, in order to maintain a givenmass flow in a given conduit, with a particular helical flow formation,the pressure drop increases as the helix angle increases and theturbulent kinetic energy decreases as the helix angle increases. Hence,the choice of helix angle is a compromise between minimising pressuredrop and minimising turbulent kinetic energy. If the pressure drop andturbulent kinetic energy are non-dimensionalised using conventionalmathematical techniques, the curves of helix angle versusnon-dimensionalised pressure drop and helix angle versus turbulentkinetic energy can be plotted on the same graph. A curve 50 of helixangle versus non-dimensionalised pressure drop and a curve 51 of helixangle versus non-dimensionalised turbulent kinetic energy for anarterial graft are shown in FIG. 2. These curves 50, 51 were obtainedfrom measuring pressure drop and turbulent kinetic energy in thearterial graft 1 using conventional techniques. The curves 50, 51 showthat at the region 52, the curves intersect and this intersection occursat a helix angle of approximately 8°.

By also analysing flow in the graft 1 using conventional magneticresonance imaging techniques it was found by trial and error that theoptimum helix angle for the graft 1 for the given mass flow was alsoapproximately 8°. Hence, the optimum helix angle for the graft 1 occursat approximately when the non-dimensionalised pressure drop isapproximately equal to the non-dimensionalised turbulent kinetic energy.

Although in the example described above the helix angle is determined asthe angle at which the non-dimensionalised pressure drop and turbulentkinetic energy are substantially equal, there may be situations in whichthe helix angle is selected so that the non-dimensionalised pressuredrop and turbulent kinetic energy are not equal. This may situation mayarise if, for example, a lower turbulent kinetic energy is required andit is decided to tolerate a higher pressure drop to obtain a lowerturbulent kinetic energy. Similarly, if a low pressure drop is moreimportant than turbulent kinetic energy, a higher turbulent kineticenergy may be tolerated to obtain a lower pressure drop. Hence, thechoice of the helix angle can be chosen according to the particularapplication, and different applications may have different requirements.

In the example described above, the helix angle of the helical formationis determined for the graft 1. However, the same technique can be usedfor other conduits where it is desired to use a helical formation toalter the flow pattern of fluid the conduit. For example, the sametechnique could also be used to determine the helix angle for a helicalformation for use in a stent, or indeed any other medical applicationinvolving the flow of a fluid through a tube.

The present invention is also suitable for industrial applications.Helical formations may also be used in conduits such as tubes to createimproved efficiency through quicker transfer of fluid and reduced energyuse or a reduction in pressure gradient along the tube allowing lowerpressures within the tube to deliver a specific end conduitpressure/flow rate. Helical formations could be used to effect areduction in turbulence, thereby reducing vibration, noise, and/orfatigue in a conduit, which in pumps could allow for reduced pump powerconsumption. Helical formations may also be used to allow furtherpenetration or more accurate distribution patterns of fluid exiting aconduit, for example from a hose pipe for domestic use or from a firehose. The invention will also be of benefit to industries where slurriesor suspensions are transported through conduits, for example foodproducers or distributors involved with soups, sauces and like products.

As with the example above of the graft 1, the optimum helix angle forthese other types of conduits can be determined from the pressure dropand the turbulent kinetic energy.

Therefore, the invention has the advantage of enabling the helix angleof a helical flow formation in a given size of conduit intended to carrya given fluid to be determined from the pressure drop and the turbulentkinetic energy in the conduit.

1. A helical formation for a conduit, the helical formation defining atleast a portion of a helix, the helical formation being made by aprocess comprising the steps of: (i) determining an angle of the helixdefined by the helical formation from the internal dimensions of theconduit, the fluid mass flow of the conduit, the pressure drop along theconduit and the turbulent kinetic energy within the conduit; and (ii)producing the helical formation with the helix angle as determined instep (i).
 2. A helical formation according to claim 1, wherein the helixangle is between 5° and 50°.
 3. A helical formation according to claim1, wherein the helix angle is between 5° and 20°.
 4. A helical formationaccording to claim 1, wherein the helix angle is substantially 8°.
 5. Ahelical formation according to claim 1, wherein the helical formation isfor effecting a rotational flow of fluid within the conduit, in use. 6.A helical formation according to claim 1, wherein the helical formationcomprises an elongate member.
 7. A helical formation according to claim6, wherein the elongate member comprises an inwardly extending portion.8. A helical formation according to claim 1, wherein the helicalformation is in the form of an insert adapted to be mounted within theconduit.
 9. A helical formation according to claim 1, wherein thehelical formation is an integral part of the conduit.
 10. A conduitcomprising a helical formation according to claim
 1. 11. A conduitaccording to claim 10, wherein the conduit is blood flow tubing.
 12. Aconduit according to claim 11, wherein the blood flow tubing comprises agraft.
 13. A conduit according to claim 11, wherein the blood flowtubing comprises a stent.